Story #69712: Further FormEngine development
Move SelectTree data providing
The SelectTreeElement currently still does the data item calculation on its own.
This should be extracted to an own data provider, probably called after TcaSelectItems / TcaSelectValues provider. Goal is that the tree element does not fetch the to-render elements on its own anymore, but instead that should be prepared by an own provider. To all this $treeDataProvider stuff should vanish from the render element.
There may be parts - especially the addItems tsConfig stuff that is already resolved within TcaSelectItems provider, so it might be a good idea to extract the according method(s) to an abstract, so both provider classes can use it. Additionally, TcaSelectItems need to be changed to not handle renderMode=tree elements.